Plasma polymerization, also known as glow discharge polymerization, is a process in which an organic substance ("monomer") is made to form a polymeric material by means of glow discharge. The glow discharge may be generated by low-frequency AC, audio-frequency, rf power, or microwave power. This type of process is typically used to synthesize new organic polymers in bulk from the vapor phase, but the technique has a much wider application for forming coatings. By selection of materials and process conditions that lead to polymers with a high degree of crosslinking, coatings that are very dense, hard, and pinhole-free can be formed.
In typical experimental method utilizing a monomer vapor, specimens to be coated are placed in a vacuum chamber which is fitted with parallel electrodes. Specimens may be placed either directly on either electrode or suspended between the electrodes. The chamber is evacuated and partially backfilled with an inert gas. Monomer vapor is then admitted to the desired pressure and power is applied to produce a glow discharge between the electrodes. The molecules of monomer vapor entering the discharge region are excited and fragmented by the field to produce highly reactive ions and free radicals. These fragments then collide with monomer molecules, both excited and unexcited, that have condensed on the substrate. The collisions excite the monomer molecules forming absorbed radicals which then combine to form long-chain, highly crosslinked polymeric films. Because of this high degree of three-dimensional crosslinking, such films can have properties significantly superior to polymeric films formed by conventional processes. The present invention retains these desirable properties in coatings which are predeposited rather than being formed in situ from a vapor.
Plasma polymerization of numerous reactive materials, including various monomers containing a functional group permitting polymerization by more conventional means, from the vapor phase into condensed films, is extensively taught in the printed literature. The printed knowledge also includes teachings of these films being deposited as plasma-formed polymer on numerous substrate materials. Illustrative teachings of plasma polymerization art can be found in "Techniques and Applications of Plasma Chemistry" by John R. Hollahan and Alex T. Bell, John Wiley & Sons, 1974, pages 191-213 under the section titled "Mechanisms of Plasma Polymerization". This section includes a Table 5.5, titled "Films Produced by Plasma Techniques", which tabulates numerous plasma-polymerizable materials, i.e. materials functioning as monomers under plasma, as well as films resulting therefrom. Included in the reported prepared films are several colors such as brown or yellow, although insofar as is known, none of the plasma-formed, colored films are prepared by the method of the present invention involving predeposited coatings and a two step curing process.